1. Field of the Invention
The present invention relates to a method of measuring the best focus; position, which is suitable for an application to a case of setting a focus position of a photosensitive substrate with respect to a projection optical system in a projection exposure apparatus employed when manufacturing, e.g., a semiconductor element, a liquid crystal display element or a thin-film magnetic head, etc. by a lithography step.
2. Related Background Art
A projection exposure apparatus is employed for manufacturing, e.g., the semiconductor element, the liquid crystal display element or the thin-film magnetic head, etc. by the lithography step. The projection exposure apparatus exposes pattern images of a photomask or a reticle (hereinafter collectively termed a [reticle]) on a photosensitive substrate through a projection optical system. In this type of projection exposure apparatus, the pattern images of the reticle are exposed on the photosensitive substrate with a high resolving power. For this purpose, the exposure is required to be done in such a manner that a focus position of an exposure face (e.g., the surface) of the photosensitive substrate, i.e., a position in the optical axis direction of the projection optical system is aligned with the best focus position, i.e., a position of the best imaging plane of the projection optical system. It is required therefor that the best focus position of the projection optical system be previously obtained by some method.
According to a known method of measuring the best focus position, as disclosed in, e.g., U.S. Pat. No. 4,908,656, the best focus position is obtained by a so-called test print in which images of focus measuring marks formed on the reticle are exposed on the photosensitive substrate while shifting a focus position. In the method of thus performing the test print, for instance, a focus measuring mark image 19P as shown in FIG. 1 is exposed on a wafer defined as a photosensitive substrate coated with a photosensitive substance while shifting the focus position. This focus measuring mark image 19P is configured such that a pattern group consists of four pieces of elongate rhombic patterns arranged at a predetermined pitch in a widthwise direction, and a plurality of pattern groups are further arranged at a pitch of 8 .mu.m.
Thereafter, the focus measuring mark image 19P after developing the wafer is transformed into resist patterns exhibiting a ruggedness. Then, a length (mark length) of each mark image 19P is measured. In this case, when the exposure face of the wafer is located in the best focus position, a length L of the focus measuring mark image 19P reaches the maximum. It is therefore possible to obtain the best focus position by measuring the mark length of the mark image 19P.
Measuring the mark length thereof has hitherto involved the following steps. An area in the vicinity of each mark image 19P is irradiated sheetwise with coherent laser beams. A wafer stage is driven while monitoring a position of the wafer stage mounted with the wafer by use of a laser interferometer. Relative scanning of the laser beam on each mark image is thus effected. When the laser beam exists on the mark image 19P, a diffracted beam or a scattered beam is produced from the mark image 19P in a predetermined direction. A length at which the diffracted beam or scattered beam is detected is measured as a mark length of the mark image 19P in the relevant focus position.
Then, the mark length has hitherto been simply approximated as a biquadratic function or thereabouts by a least squares method. A focus position in which the function comes to the maximum value within a measuring range is set as the best focus position.
When measuring a size of the focus measuring mark image by use of coherent beams such as the laser beams as done in the prior arts, an intensity of a detection signal which is obtained by photoelectrically converting the beams from the mark image varies depending on a profile of the mark image. Accordingly, it happens that the mark length is measured larger or smaller than an actual mark length, depending on a degree of interference of the beams from the mark image. A further drawback is that the calculated best focus position largely shifts due to an exposure quantity or a thickness of a resist film applied on the wafer when effecting the exposure.
For example, FIG. 2 shows a result of approximating the mark length actually measured by the conventional method with a biquadratic function of a focus position F. Referring to FIG. 2, a curve 29 corresponds to a function for approximating the mark length obtained by shifting the focus position at a pitch of 0.3 .mu.m when an exposure energy is a fiducial energy. A curve 30 corresponds to an approximation function when the exposure energy is smaller by 10% than the fiducial energy. A curve 31 corresponds to an approximation function when the exposure energy is larger by 10% than the fiducial energy. Referring again to FIG. 2, if, for example, the exposure energy is larger by 10% than the fiducial energy, as seen from the curve 31, measurement data of the mark length contains data of particularly a large value. Hence, if a peak position of the curve 31 is simply conceived as the best focus position, the calculated best focus position largely deviates from the true best focus position, resulting in an error in terms of measurement.
Further, in the case of FIG. 2, an exposure time of the focus measuring mark image is changed. When shifting such a focus position that the measurement error of the mark length due to the interference by the laser beams becomes plus (+) and such a focus position that the measurement error becomes minus (-), the best focus position obtained by the conventional method shifts by the order of 0.1 .mu.m from the one in FIG. 2. Besides, the approximate curve is to be obtained even in the state where the exposure energy is larger by 10% than the fiducial energy by eliminating the data of the mark length extraordinarily larger than the one in FIG. 2. Obtained in this case are biquadratic approximation functions expressed by curves 29A-31A as shown in FIG. 3. According to characteristics of FIG. 3, however, a fluctuation width of the best focus position with respect to the changes in the exposure energy is on the order of 0.6 .mu.m. No stable measurement result is consequently obtained.
Moreover, if the intensity of the detection signal from the focus measuring mark image decreases in the conventional example, noises intrinsic to a measuring sensor disorders the detection signal. It may happen that the size of the mark image is measured remarkably long in some cases. This conduces to a drawback in which the best focus position is miscalculated or alternatively can not be calculated.